I got my whistle today, and I have had time to start playing it tonight. Now, I am a super beginner. Just learning, and going through a book. The notes sound good until I get to C# and D'. They sound awful. Is this me? How can I fix this?

Without knowing why they 'sound awful' there's little to say about this but try blow just a steady C#, if need be support the whistle by covering one or two lower holes OOOOXX and blow a steady stream of air until you get a reasonably satisfying tone. Then move on to d' OXXXXX, making sure you cover all holes properly. Make sure you're in the lower octave for C# and hit second d' rather than starting at the top of the second octave and trying to hit third d.

The Mellow Dog D is a very well respected, much loved tweaked whistle. Practice and patience will move you forward. Check out YouTube for people playing various whistles, including the Mellow Dog. Most whistles are just fine. It takes lots of practice.

It is not uncommon to have some notes sound dreadful when you are beginning. There is a lot you will learn about breath control as you progress that will help make all the notes sound like music. Keep working and be patient with yourself.